Roughly half of the recently approved FDA pharmaceuticals are derived from natural products, molecules that are produced by microorganisms that enable them to adapt to diverse environments. One class of important natural products are produced by the Nonribosomal Peptide Synthetases (NRPSs), a family of large proteins that contain multiple catalytic domains joined in a single modular protein. Through structural and functional studies of multidomain NRPSs, we have sought to understand the features that guide the efficient biosynthesis of these important enzymes. We have also recently explored the structural basis for catalysis of unusual reactions catalyzed by NRPS proteins involved in the production of antibiotics and peptide siderophores.